The invention concerns a method and an apparatus for precooling the helium tank of a cryostat, in particular, an optical cryostat with optical components in the helium tank, an NMR cryostat, or a medical NMR cryostat for magnetic resonance imaging which accepts a superconducting coil.
Cryostats which can accept a superconducting magnetic coil are, by way of example, known in the art from the area of nuclear resonance spectroscopy or magnetic resonance imaging (DE-OS 29 06 060, DE-PS 37 24 562). These cryostats exhibit a plurality of containers or tanks which are nested within each other and the innermost one contains a magnetic coil which, during operation, is filled with liquid helium at a temperature of approximately 4 K. An outer tank contains fluid nitrogen with a temperature of approximately 77 K. Both tanks are vacuum isolated with respect to each other and with respect to room temperature, whereby the evacuated intermediate space contains radiation shields at intermediate temperatures so that both the heat transport as well as the heat radiation is minimized. Before the initial operation or following a maintenance or repair on the magnetic coil or on the cryostat, said cryostat must be cooled down to the operating temperature. Many prodecures are known in the art by which the cryostat can be cooled down to the operating temperature. One procedure proposes filling the nitrogen tank with fluid nitrogen and the helium tank with fluid helium, and to cool down in this manner. Towards this end, however, large quantities of fluid helium are necessary since the magnetic coil and the inner space of the cryostat must be cooled down with the fluid helium only. The liquid helium, despite its low temperature of 4 K, exhibits only a small capacity for heat absorption compared to that of liquid nitrogen at a temperature of 77 K. In consequence, in addition to the large quantity of fluid helium used which, in addition, is very expensive, there is a sizeable time expense due to the small heat absorption capability. These disadvantages are, furthermore, not justified by the requirement that the helium tank come solely in contact with the helium cooling medium in order to avoid impurities, in particular, when the tank contains elements with large heat capacities, for example, a superconducting magnet coil.
Another method proposes venting the evacuated intermediate space between the nitrogen and helium tanks by means of dry nitrogen gas so that the helium tank can be cooled down to the temperature of 77 K via a heat conduction. This procedure has, however, the disadvantage that the nitrogen must then be removed from the evacuated intermediate space. Since, in general, this can only be partially achieved, the remaining nitrogen freezes onto the cooled helium tank when the cryostat is later put into operation. Furthermore, there are possible vacuum valve operational faults which present, in turn, safety risks, since there is the danger of an explosion as a result of the overpressure coming from the evaporating condensed nitrogen or air. Moreover, this indirect cooling by way of the intermediate evacuated space between the nitrogen tank and the helium tank is ineffective, time consuming, and in addition, in the event of one single interconnected intermediate vacuum space between the nitrogen tank and the helium tank on the one hand and the ambient environment on the other hand, leads to an icing of the outer jacket of the cryostat.
Another and the most common method is to initially fill the helium tank with liquid nitrogen, whereby the tank is cooled down to a temperature of 77 K. In consequence of this cooling down, most of the total heat is removed from the cryostat. However, the liquid nitrogen must then be completely removed from the helium tank following the cooling down procedure. Remaining quantities of nitrogen cause reduction in the helium storage time, which, with NMR magnets, corresponds generally to one year, and, in particular, remaining quantities of nitrogen on the magnet coil cause a reduction in the operating safety of the magnet coil, that is to say, an increased risk of a quench, that is to say, an unintentional transition from the superconducting to the normally conducting state of the magnet coil. Furthermore, with optical cryostats, there is the danger that optical components such as windows, mirrors or other optical elements, become coated with frozen nitrogen. If the nitrogen exhibits a certain fraction of oxygen, that is to say is contaminated with oxygen, then this paramagnetic component will be attracted to the magnet coil during operation. Furthermore, there is the additional increased danger that in consequence of the disassembly needed during the filling procedure or the exchange of two liquid gases, that ambient air and, thereby, moisture can condense, which, in turn, can lead to operational failures and reduce the storage time of the helium. Furthermore, there is an increased safety risk associated with operational error.
The positioning of an additional nitrogen tank on the outside of the helium tank which coaxially surrounds the helium tank is also known in the art of cryostats. In this arrangement the nitrogen tank can then be filled with fluid nitrogen, whereby, the wall of the helium tank and thereby the inside of the helium tank are gradually cooled down. This method exhibits, however, the disadvantage that, within the helium tank, an insufficient amount of convection of the helium gas within the helium tank is produced due to the symmetric arrangement of the nitrogen tank around the helium tank. This symmetric arrangement is, however, essential for an even weight distribution of the tank in the cryostat. Furthermore, there is an additional substantial disadvantage in that the entire volume taken in by this nitrogen tank does not contribute to the cooling during the subsequent steady state operation of the cryostat. Even if the entire nitrogen were removed from the tank after precooling of the cryostat and this tank then filled with helium, the helium would not contribute to the cooling of the superconducting magnet, since, the helium in this tank is confined and cannot contribute to the convection and heat transfer in the actual helium tank of the cryostat.
One can therefore in general conclude that, with the cryostat configured in this manner, the space within the cryostat is not optimally utilized.
Therefore, an object of the invention is to provide a method and/or an apparatus for the precooling of a helium tank of a cryostat in which the above mentioned disadvantages are avoided.